| Summary: | 碩士 === 國立臺灣師範大學 === 生命科學研究所 === 97 === The mechanisms of Na+ uptake and NH4+ excretion at gills of freshwater fish have been studied for decades but the detail remains unclear. To investigate the mechanisms, a scanning ion-selective electrode technique (SIET) was applied to detect the H+, Na+, and NH4+ activities and fluxes at the skin surface of newly-hatched medaka larvae. By probing the ionic fluxes at specific cells in the skin, MRCs were found to be the major sites for Na+ uptake and NH4+ excretion. However, H+ probing at MRCs revealed two groups of MRCs: acid-secreting MRCs (MRC+) and base-secreting (probably NH3) MRCs (MRC-). Treatment with EIPA (100 μM) respectively blocked H+ excretion, NH4+ excretion, and Na+ uptake by 22%, 35%, and 54 %, suggesting that the Na+/H+ exchanger (NHE) is involved in H+, Na+, and NH4+ transport. Low-Na+ water (< 0.001 mM) or high-NH4+ water (5 mM) acclimation caused more MRC- appearing in skin surface, and simultaneously increased Na+ uptake and NH4+ excretion but decreased or even reversed the H+ gradient at the skin and the H+ flux at MRCs. Raising the external NH4+ significantly blocked NH4+ excretion and Na+ uptake, but increased the H+ gradient at the skin. In contrast, raising the acidity of the water (pH 7 to pH 6) enhanced NH4+ excretion and Na+ uptake by MRCs while the H+ activity at the apical surface of MRCs was reduced. The correlation between NH4+ production and H+ consumption suggests that MRCs excrete non-ionic NH3 (base) by an acid-trapping mechanism. The present study suggests a Na+/NH4+ exchange pathway in apical membranes of MRCs, in which a coupled NHE and Rhesus (Rh) glycoprotein is involved, and the Rh glycoprotein may drive the NHE by generating H+ gradients across apical membranes of MRCs.
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